Productivity and profitability of paper making is determined by the speed of production, that is, the speed with which the paper web progresses through the paper making and paper processing equipment. Production speeds may be as high as 4000 ft/min, but 5000 ft/min or higher would obviously be more profitable. So-called web breaks seriously limit production for two reasons. First, a web break stops production for up to 45 minutes causing a loss of 45.times.4000=180,000 ft of production (up to 6 tons of paper). Up to 6 web breaks may occur in 24 hours. Second, the higher the production speed, the more web breaks occur, so that production speed is limited by the number of web breaks.
Paper is produced as a continuous sheet of a width often greater than 20 feet. This continuous sheet is commonly referred to as the `web`. At the end of the machine the paper is wound on a roll. When a roll has reached a certain size, the web is cut on-the-fly, and a new roll is wound automatically. The rolls so produced are called `logs`. In line with the paper machine is the re-reeler in which the logs are rewound. The purpose of the re-reeler will be explained in the following. The logs coming from the re-reeler are fed into the coater, a machine several hundred feet in length in line with paper machine and re-reeler. In the coater the paper is coated, often on both sides, usually with a clay-based material, primarily to improve printability. The initial web coating must be dried, the other side coated and dried and the final product wound up in new logs. Web breaks in the coater are of concern here. If the web breaks paper is spewed all over at 4000 ft/min. The machine has to be stopped and rewound with the associated production loss as explained above. These web breaks are caused by defects in the paper introduced in the paper machine. Control in the production process in the paper machine must detect those defects. In the re-reeler these defects are repaired if serious enough. But repairs are costly and time consuming. In one aspect of the invention proposed here will automatically identify those defects that warrant repair, mark them and make the re-reeler stop automatically at the defect so that a repair can be made and, more important, automatically decide which defects should be repaired, and which should not, depending upon the chance the defect would cause a web break. This allows optimization of production. Alternatively, another aspect of the invention enables those defects that most warrant repair to be marked, such as by automatically marking the paper web in the region of the defect, so that a machine operator can stop movement of the paper web at the re-reeler and repair the marked defect if desired.
As used herein, the terminology "through web defect" refers to any defect which passes completely through the thickness of the paper web such as cracks, circular holes, elliptical holes, and irregular holes. The terminology "web defect" refers to both through web defects and other types of defects including, but not limited to light spots and dark spots caused by significant variances in thickness of the paper web and/or clumps of material. As pointed out in applicant's paper entitled Tenacity, Fracture Mechanics and Unknown Coater Web Breaks TAPPI J. 79(2) Kovalin 233 (1996), such through web defects reduce the strength of the paper web in the region of the defects permitting failure or breaking of the paper web in the region of such defects at a lower tension. The advantage of using fracture mechanics to determine the failure strength of through web defects is likewise described in the subject paper.
Accordingly, a system for real time monitoring of web defects combined with a method to evaluate which defects should be repaired, will be of great benefit.